There is a class of sweeping machines which contact the floor or ground being swept with a cylindrical brush that lifts debris from the surface and throws it forward directly into a debris hopper located in front of the brush. Such machines are referred to as direct forward throw sweepers, and it is sometimes said that they use a "broom and dustpan" sweeping principle. The debris hopper of such a machine is open at the rear for entrance of debris, and the hopper floor is set close to the ground, at least in the entrance area. A rubber lip is commonly attached to the rear edge of the hopper floor and made so it drags on the ground, so the hopper is in fact built somewhat like a dust pan, and the rotating broom sweeps debris into it. U.S. Pat. No. 3,189,931 (Peabody) and U.S. Pat. No. 3,304,572 (Wendel) show representative sweepers of this class. In this discussion we will refer to the sweeping principle used in such machines as the conventional sweeping mode.
Such sweepers have been used for many years, and their operating characteristics are well known. They are recognized as being extremely efficient in sweeping fine, dense debris such as sand and gravel. Starting from ground level, they throw such material in a low trajectory well forward in the hopper and easily load the hopper to its capacity. However, they do less well in sweeping and hopper loading of light debris such as, for example, crumpled paper items or dry leaves. This is primarily because air resistance checks the flight of light debris to the front of the hopper. Much of it falls in the rear of the hopper, where it builds up and blocks the hopper entrance before the hopper is full.
In the mid '80's a two-tool sweeper design emerged which was much superior in loading light debris. Shown in U.S. Pat. No. 4,624,026 (Olson), it used the conventional sweeping brush, but in addition a smaller cylindrical brush or paddle wheel was placed in front of the brush so it just cleared the ground and was rotated opposite to the sweeping brush rotation. These two tools cooperatively threw debris in a much higher trajectory than direct forward throw sweepers. In this trajectory the debris entered the hopper at a higher level than before. Even light debris travelled farther forward in the hopper before it came to rest, so almost a full hopper load of it could be collected. Sweepers built to this design were outstanding in their ability to sweep and hopper load light debris.
However, they did not sweep sand and gravel as well as the direct forward throw sweepers. Sand, when thrown by a sweeper brush, fans out to some degree, like a shotgun pattern. This did not affect the low trajectory of the direct forward throw sweepers, but in the high trajectory of the two-tool sweepers a small portion of the sand fanned out enough to fall back into the top of the sweeping brush rather than flying forward into the hopper. The rotating brush carried it backward and dropped it behind the brush, where it could not be swept up. Only a small percentage of the sand was lost in this way, but it was enough to create dissatisfaction with sweeper operation.
A need exists for a sweeper that will sweep dense debris such as sand and gravel as efficiently as a conventional direct forward throw sweeper, and also will sweep and hopper load light debris such as crumpled paper items or dry leaves as efficiently as a two-tool sweeper. A mixture of dense and light debris should also be efficiently swept and hopper loaded.